Electric blasting cap having heatshrinkable plastic-covered match assembly



July 25, 1967 A. E. MORFY 3,332,350

ELECTRIC BLASTINC- CAP HAVING HEAT-SHRINKABLE PLASTIC-COVERED MATCH ASSEMBLY Flled Oct 14 1965 2 Sheets-Sheet l FIG. 2

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INVENTOR. Anthony E. Morfy BY July 25, 1967 A. E. MORFY 3,332,350

ELECTRIC BLASTING CAP HAVING HEAT-SHRINKABLE PLASTIC-COVERED MATCH ASSEMBLY FIG. 3

INVENTOR. Anthony E. Morfy United States Patent ABSTRACT OF THE DISCLOSURE In the art of electric blasting caps provided with an electric match assembly, the electric match assembly is provided with a heat shrinkable plastic envelope along the entire length thereof so as to cover the entire match assembly except for the tip end of the match head closest to the flash charge. Blasting caps are thus prepared which fire within a very narrow time period of each other.

The present invention relates to electric blasting caps which have an increased speed of initiation.

The electric blasting caps which are the subject of this invention are of the match type which are comprised of a metallic shell having a detonating base charge disposed in one end of the shell and a pair of leg wires terminating within the opposite end portion of said shell which are connected to a match assembly. The leg wires are connected to the match assembly at points on metal match tabs which are suitably of a trapezoidal shape, having a narrow bottom portion. A layer of non-conducting material electrically separates the said metallic tabs. A high resistance bridge wire electrically connects the said match tabs along'the narrow bottom portions. Surrounding the bridge wire and in intimate contact therewith is a match composition forming a match head. This match head is suitably formed around the bridge wire. Suitable materials for the match head' ignition composition comprise, for example, cuprous acetylide, lead mononitrorescorcinate, diazodinitrophenol, mixtures of lead mononitroresorcinate and potassium chlorate and silver and lead azides.

A primer charge is positioned adjacent to the base charge so as to be in detonating relation to the base charge and to a flash charge. The primer charge may be comprised of any suitable material such as mannitol hexanitrate or diazodinitrophenol. The base charge is comprised of a suitable detonable material, for example, pentaerythritol tetranitrate, tetryl or the like. A flash charge is positioned atop the primer charge immediately adjacent the match head. The flash charge may be comprised of any suitable material, for example, 75% by weight of diazodinitrophenol and 25% by weight of mannitol hexanitrate.

It is known that a multiplicity of electric blasting caps such as described above on receiving an electrical charge suflicient to detonate the blasting caps generally will fire over a period of a fraction of one second. They normally will not all fire at the same instant. The variation and lag in detonation of the subject caps occurs because of the variation in induction period which is the period between the application of electric current to cause ignition of the match head and the actual detonation of the blasting cap base charge. The induction period will generally vary between blasting caps of the same manufacturer due to the manner in which the match head within any particular cap burns and the direction in which its flame is directed.

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In the seismograph trade blasting caps are considered suitable if in five shots comprised of 10 electric blasting caps each, the average spread or elapse of time in firing each of the five shots is .00075 second or less with no individual shot having a spread of firing time greater than .0015 second as measured between the firing of the fastest cap and the firing of the slowest cap.

In many explosive operations such speed of firing and variations of firing time between caps after initation is satisfactory. However, in some applications, for example in seismographic exploration for mineral and other deposits, the necessary record of the exact instant of detonation and the necessity of having the caps fire within a close time of each other is extremely important in order to obtain meaningful results.

In accordance with the present invention it has been discovered that by enveloping the entire length of the match of a blasting cap in a plastic cover so as to cover the entire match assembly except for the tip end of the match head closest to the flash charge it is possible to produce blasting caps which fire Within a very narrow period of each other.

The plastic covers of the present invention which are positioned over the subject match assembly may be of any suitable design which can be applied over the match assembly so as to enclose the Whole assembly in a skin tight fashion except for the tip end of the match head. Commercially available heat shrinkable tubing may be used to provide the coating of the present match assembly. For example, a tube of material having a diameter of about .150 inch and about /3" long may be placed over the match and then shrunk around the match by applying heat of approximately 375 F.; however, the size and type of material can be suitably varied to accommodate the type and size match assembly being utilized. Conventional means such as a hot air gun Orheating element can be used for heat source. Such tubing when shrunk will assume the shape of the match assembly. Various plastic materials maybe used for the covering of this invention so long as they have a heat shrinkage of from about 30-50%. Examples of suitable materials are irradiated polyethylene, irradiated polyvinyl chloride and irradiated rubber.

In the evaluation of the blasting caps of the present invention groups of ten electric blasting capsare connected in a single series to a firing device such as a blasting machine which can provide a suitable electric current to initiate the blastaing caps to be tested. Connected to a timing device such as a Hewlett-Packard electronic timer, sold by the Hewlett-Packard Co. of Palo Alto, Califl, are a series and a parallel circuit. The wiring of these two particular circuits is so arranged that'the wires wraparound and cross each of the ten blasting caps in the test group. The following sequence occurs during the test:

(l) The blasting machine is activated and in turn initiates the ten blasting caps connected in series.

(2) When the first cap fires it breaks the series circuit connected to the timing device and starts the electric timer.

(3) When the last cap in the test series fires, the final portion of the electric timers parallel circuit is broken which in turn acts to stop the electric timer.

(4) The lapse of time between the first and last cap to fire is then read directly from the timer.

Some typical times obtained with the caps of the present invention are set forth in the following 'Table I wherein the time of each shot represents the time lapse between the firing of the fastest and the slowest electric blasting cap of that particular shot.

TABLE I Test 1 Shot No.: Time, seconds 1 .0003172 2 .0002052 3 .0004884 4 .0005564 5 .0004832 Total .0020504 Average .0004101 Test 2 Shot No.: Time, seconds 1 .0008032 2 .0006344 3 .0005032 4 .0004756 5 .0003612 Total .0027776 Average .0005555 Test 3 Shot No.: Time, seconds 1 .0003208 2 .0002228 3 .0003944 4 .0005304 5 .0001424 Total .0016108 Average .0003221 Some typical times obtained with conventional electric blasting caps wherein the match assembly is not enveloped with the plastic cover of the present invention are set forth in Table II.

In order to describe the invention more clearly, reference is made to the accompanying drawings wherein one embodiment of the present invention is illustrated and where similar numerals denote similar components throughout the several views.

FIGURE 1 is an enlarged front elevation partially in section of an electric match assembly in accord with the present invention.

FIGURE 2 is an enlarged side view in section of the match of FIGURE 1.

FIGURE 3 is a sectional elevational of an electric blasting cap containing a match of the present invention.

FIGURE 4 is an enlarged end view of the head end of the match of FIGURE 1.

Looking now in detail at the drawings and in particular to the match assembly shown in FIGS. 1 and 2. Metallic match tabs 15 and 17 are trapezoidal in shape having narrow portions 37 and 39 respectively. A layer of non-conductive material 19 electrically separates tabs 15 and 17. A bridge wire 21 electrically connects match tabs 15 and 17 at tab portions 37 and 39. Surrounding bridge wire 21 and in intimate contact therewith a match composition forming match head 23. Plastic tube 25 tightly envelopes the entire length of match assembly 38 leaving the tip of match head 23 uncovered as shown in FIGURES 1 to 4 and exposed in the direction of flash charge 31 as shown in FIGURE 3.

In FIGURE 3 shell 35 is comprised of a metal such as copper, brass, aluminum, ferrous metals and various other alloys and at the bottom end contains a base charge 27, primer charge 29 and flash charge 31. A perforated cup 32 is mounted in an inverted position above the charges. Adjacent to charge 31 is a match assembly 38 as shown in FIGURES 1 and 2. Leg wires 11 and 13 are connected to match assembly 38 and extend out of shell 35 through rubber plug 33.

What is claimed is:

1. An electric blasting cap, which comprises in combination, a match assembly and a heat shrunk plastic means covering the length of said match assembly in intimate contact therewith so that a plurality of said blasting caps so prepared fire within a very narrow time period of each other when connected in a single series to a firing device.

2. The electric blasting cap of claim 1 wherein said heat shrunk plastic means is a plastic cover enveloping the length of said match assembly so that the match head end portion is exposed.

3. The electric blasting cap of claim 1 provided with a detonating charge and wherein said plastic means is a plastic tube tightly enveloping the length of said match assembly so that the match head end portion is exposed in the direction of said detonating charge.

References Cited UNITED STATES PATENTS 2,350,172 5/1944 Lawrence 102-28 2,506,229 5/1950 Lowe et al 102-28 2,549,533 4/1951 Sevold 102-28 BENJAMIN A. BORCHELT, Primary Examiner.

SAMUEL FEINBERG, Examiner.

V. R. PENDEGRASS, Assistant Examiner. 

1. AN ELECTRIC BLASTING CAP, WHICH COMPRISES IN COMBINATION, A MATCH ASSEMBLY AND A HEAT SHRUNK PLASTIC MEANS COVERING THE LENGTH OF SAID MATCH ASSEMBLY IN INTIMATE CONTACT THEREWITH SO THAT A PLURALITY OF SAID BLASTING CAPS SO PREPARED FIRE WITHIN A VERY NARROW TIME PERIOD OF EACH OTHER WHEN CONNECTED IN A SINGLE SERIES TO A FIRING DEVICE. 